In some air-cooled engines, the cylinder head is provided with a valve chamber for accommodating an intake valve and an exhaust valve, and the cylinder head is superposed on the cylinder block and is fastened on with bolts. This type of air-cooled engine is disclosed in Japanese Examined Utility Model Application No. 2-32849.
The air-cooled engine disclosed in Japanese Examined Utility Model Application No. 2-32849 is a multipurpose engine wherein a cylinder head provided with a valve chamber and a cooling air duct is superposed on and bolted to the cylinder block. The cylinder head includes three mounting holes disposed inside the valve chamber (valve compartment), and two mounting holes disposed outside the valve chamber. Bolts passed through these five mounting holes are screwed into the cylinder block, whereby the cylinder head can be attached to the cylinder block.
Lubricating oil is supplied to the interior of the valve chamber. Sufficient care must therefore be taken to prevent the lubricating oil from leaking through the mounting holes of the bolts inside the valve chamber. For example, oil leakage can be prevented by means of a gasket (seal member) having a complicated shape placed between the cylinder head and the cylinder block.
There is a large difference between the temperature of the bolts provided inside the valve chamber and the temperature of the bolts provided outside the valve chamber. Care must therefore be taken to maintain uniform thermal strain in the interior and exterior bolts. Furthermore, space is needed inside the valve chamber to allow the three bolts to be accommodated, and the valve chamber must be enlarged accordingly. Reducing the size of the air-cooled engine is therefore limited. It is apparent that the cylinder head constitutes part of the combustion chamber of the engine. The valve chamber is provided so as to cover part of the combustion chamber in the cylinder head. Therefore, with a large valve chamber, part of the combustion chamber is covered by the valve chamber, which impedes the cooling air in its ability to reach the vicinity of the combustion chamber.
In view of this, there is a need for techniques whereby oil leakage from the valve chamber can be prevented, the thermal strain in the bolts for securing the cylinder head can be made uniform, the engine can be reduced in size, and cooling air can be conducted to the vicinity of the combustion chamber.